1. Field
Apparatuses and method consistent with exemplary embodiments relate to a fuel injection system and a method of controlling the same, and more particularly, to a fuel injection system that may stably fuel an auxiliary power unit (APU) for an aircraft while an altitude of the aircraft changes and a method of controlling the fuel injection system.
2. Description of the Related Art
Auxiliary power units (APUs) for aircrafts supply power to generators, hydraulic pumps, or air-conditioning and heating equipment mounted in the aircrafts when the main engines of the aircrafts break down or before the main engines are started (i.e. before the aircrafts take off).
Generally, an APU is a small gas turbine engine or the like. Although the APU supplies emergency power necessary for starting the main engines or for a safe flight, the APU has an operational limit according to a change in a flying altitude of an aircraft.
For example, civil aircrafts may fly at an altitude of approximately 11 km (36,000 feet) above the ground. At this altitude, air pressure decreases about 23% compared to the air pressure on the ground, and therefore, a maximum fuel flow rate needed to operate an APU also decreases up to about 20% as compared to the flow rate need on the ground. A minimum fuel flow rate needed to operate the APU when starting and running the APU with minimum power is designed to be about 5% of the maximum fuel flow rate on the ground. However, in order for the APU to stably operate even at 13 km (41,000 feet), which is the highest flying altitude of a civil aircraft, the APU has to be able to work with a flow rate even less than the minimum fuel flow rate, which is about 3 to 4% of the maximum fuel flow rate on the ground.
The highest flying altitude of military aircrafts reaches 18 km (55,000 feet). At this altitude, air is rarefied and thus most APUs are impossible to start and operate.
Such a limit to starting and operating an APU as described above exists because a range of fuel flow rate needed to operate the APU also varies greatly as an altitude of an aircraft changes from the ground to the highest flying altitude of the aircraft. Generally, a fuel nozzle that fuels an APU combustor is composed of a main nozzle and a pilot nozzle having different flow rates from each other. Due to the fuel nozzle having two nozzles, it is difficult to fuel the APU combustor while maintaining excellent fuel injection characteristics in all the fuel flow rate ranges which correspond to the aircraft's changes in the flying altitude.
U.S. Pat. No. 4,543,785, filed on Oct. 1, 1985, introduces a technology of dividing a combustion chamber into a main burner and a secondary burner and injecting fuel into a low-temperature portion by using a lower nozzle in order to operate an aircraft engine at a high altitude. Although the intention of this technology is to improve the total efficiency of the engine, the problem of a change in the fuel flow rate of an APU according to a change in an altitude of an aircraft is not addressed by the technology.
Korean Patent Application Publication No. 2012-0028113, filed on Mar. 22, 2012, discloses an apparatus and method of controlling the number of pilot injections. The technology enables injecting fuel into an engine by varying the number of pilot injections according to an operation condition of the engine, thereby responding to an acceleration condition of the engine. However, the problem of a change in fuel flow rate of an APU according to a change in an altitude of an aircraft is not addressed by the technology.
U.S. Pat. No. 3,931,808, filed on Jan. 13, 1976, discusses a technology of adjusting the amount of fuel supplied to an engine as a system for compensating for a change in an altitude of an aircraft. Such a technology employs a method of adjusting an injection cycle of an injector by sensing a change in ambient atmospheric pressure according to an altitude change and altering a pulse width of a control signal supplied to the injector. However, as a flow rate range injected by one injector is limited, it is difficult to solve a fuel flow shortage problem of an APU that occurs at high altitude through the method of adjusting an injection cycle of an injector.
Japanese Patent Application Publication No. 2008-274874, filed on Nov. 13, 2008, discusses a technology of compensating for a change of altitude based on a drop in density of intake air. However, as such a technology is for reducing the amount of fuel injection, it is difficult to solve the fuel flow shortage problem of an APU that occurs at high altitude.